1. Field of the Invention
The present invention relates to an ultrasonic diagnostic system in which ultrasonic beams are transmitted into the inside of the subject and the ultrasounds reflected from the inside of the subject are received, thereby deriving for example, a velocity, a velocity Gradient or the like of a tissue in the subject on the basis of the received signal obtained through reflection.
2. Description of the Related Art
There has been used an ultrasonic diagnostic system for transmitting ultrasonic beams within the human body and receiving the ultrasounds reflected by a tissue in the human body thus diagnosing diseases of the viscera and the like of the human body. In one aspect of this ultrasonic diagnostic system, or in an optional function of an ultrasonic diagnostic system for displaying a tomographic image (B-mode), there has been known an ultrasonic diagnostic system having a function of detecting a velocity of the blood flow in the human body and a movement of the tissue.
In such type of ultrasonic diagnostic system, there is known a system which is arranged in such a manner that ultrasounds reflected by blood flow within the human body are received to obtain blood flow information such as velocity, variance, power and the like of the blood flow. Further, recently, for example, for the purpose of serving diagnostic of ischemic heart disease such as angina pectoris and myocardial infarction, or for the purpose of discovering a hard cancer tissue or the like within a tissue, it is proposed to observe movement and hardness of the cardiac muscle and other tissues. It is possible to know the movement and hardness of these tissues through observation of properties of propagation of vibration in application of an external vibration to a living body, a movement of the internal organs due to the internal heart beat, and the like using ultrasound beams.
With respect to a technique for detecting a quantity associated with hardness of a tissue, for example, in "Simultaneous video system of amplitude and phase of vibration within soft tissue by application of low frequency vibration", pp. 271-272, Proceedings of the 53rd Meeting of the Japan Society of Ultrasonics in Medicine, published on November, 1988, it is proposed to determine from a phase variation a propagation speed of a low frequency vibration of a tissue inside the subject as a physical quantity closely involved in a shear viscoelasticity parameter associated with hardness, elasticity and the like of the tissue.
Further, recently, it is proposed not only to detect a movement (velocity) of a tissue, but also to obtain a quality (hardness) associated with a degree of shrinkage of the tissue through determining a velocity gradient by means of differentiating the detected velocity with respect to, for example, a depth direction within the subject (cf. for example, Japanese Patent Publication No. 43381/1993).
It is possible to detect a blood flow velocity within the subject and a movement of an tissue of the subject in such a manner that ultrasound pulses are transmitted plural number of times in each scan line direction extending inside the subject, and a Doppler shift frequency is detected in accordance with a pulse-pair method, whereby the blood flow velocity and the movement of the tissue are detected on the base of the detected Doppler shift frequency.
FIG. 37 is a block diagram of a related art of ultrasonic diagnostic system in which a blood flow velocity is detected.
A transmitter unit 110 transmits a transmission signal to a transducer array 112, which is arranged in the tip of an ultrasound probe, in accordance with a control signal from a control unit (not illustrated), so that ultrasounds are transmitted from the transducer array 112 to the inside of the subject (not illustrated). The ultrasounds reflected inside the subject are received by the transducer array 112. The received signals are applied to a beamformer 114 by which echo signals converged into a specified direction are generated. The specified direction is referred to as a scan direction. The echo signals generated from the beamformer 114 are applied to a quadrature detector 116 in which the echo signals are subjected to a quadrature detection in accordance with two reference signals each having the same frequency as a central frequency of the ultrasounds and mutually different by 90.degree. in phase, which reference signals are transmitted from the control unit (not illustrated), so that real part R.sub.j (i) and imaginary part I.sub.j (i) of two signals are built on each echo signal. Where the sign "i" is used to denote the scan line number, and the suffix "j" to denote the repeated number which is applied to transmitting and receiving of a plurality of number of times for ultrasounds conducted with respect to the same scan direction.
The real part R.sub.j (i) and imaginary part I.sub.j (i) of quadrature detected signals, which are derived from the quadrature detector 116, are converted by A/D converters 118 and 120 into digital signals, respectively, and then temporarily stored in memories 122 and 124, respectively. Thereafter, the digital signals stored in the memories 122 and 124 are read out and supplied to an MTI filter 126. Since the signals entered the MTI filter 126 carry both blood flow information and additional information as to a movement of a tissue of a living body, signals carrying only the blood flow information can be extracted through eliminating by the MTI filter 126, for example, a low frequency component of the received input signal.
An output signal of the MTI filter 126 is transmitted to a complex auto-correlation arithmetic unit 128, and of which an output signal is transmitted to an atan arithmetic unit 130. The arithmetic contents in those arithmetic units will be described later.
The atan arithmetic unit 130 serves to determine a blood flow velocity. A signal outputted from the atan arithmetic unit 130, which is representative of the blood flow velocity, is applied to a digital scan converter 132 to be converted into a signal suitable for display in a display 134, for example, a CRT in which an image representative of the blood flow velocity is color-displayed, for example, in such a manner that a blood flow running in a direction from the inside of the subject toward the transducer array 112 is color-displayed with red, and a blood flow running in the reversed direction with blue.
FIGS. 38(A)-38(C) are each a typical illustration of scan lines within the subject in a case where a blood flow velocity is detected. FIGS. 39(A)-39(D) are each a diagram useful understanding the operation of the related art. FIG. 40 is a flowchart of arithmetic algorithm of the related art.
As shown in FIGS. 38(A)-38(C), transmitting ultrasounds from the probe contacted with the body surface in the scan line directions depicted with B1, B2, . . . B5 permits echo signals for obtaining a B mode of tomographic image to be derived. Further, in a case where a blood flow velocity is detected, transmitting ultrasounds in the scan line directions depicted with C1, C2, . . . C5 each plural number of times permits a blood flow velocity on each scan line direction to be calculated in accordance with a pulse-pair method. While FIGS. 38(A)-38(C) show each that the scan line directions B.sub.i and C.sub.i are mutually different form each other, it is acceptable that they are of the same.
Generally, a sequence of the scan is given in a fashion as shown in FIGS. 39(A) or 39(B). A signal for detecting a blood flow velocity or a movement of the tissue is denoted by C.sub.j (i), where the suffix "j" is used to denote the scan line number, and the sign "i" to denote the repeated number which is applied to transmitting of a plurality of number of times for ultrasounds conducted with respect to the same scan direction. In general, if it is desired to detect a higher speed of blood flow velocity as well, there is provided a short repetitive period as shown in FIG. 39(A), and on the other hand, if it is desired to detect a lower speed of blood flow velocity as well, there is provided a relatively long repetitive period as shown in FIG. 39(B). Where quadrature detected signals, which are repeatedly generated with respect to the scan line j direction, are expressed as {R.sub.j (1), I.sub.J (1)}, {R.sub.j (2), I.sub.j (2)}, {R.sub.j (3), I.sub.j (3)}, . . . , as shown in FIG. 39(C).
Whereas, in the complex auto-correlation arithmetic unit 128 and the atan arithmetic unit 130, the arithmetic operations as shown in FIG. 40 are performed.
When an image on a screen is updated, first, an initial setting is conducted with respect to the scan line number j and depth t, (corresponding to time t.sub.i shown in FIG. 38 (C)). Quadrature detected signals involved in the scan line of scan line number j and the designated depth t.sub.i, which are expressed by EQU {C.sub.j (i)}.sub.t=ti ={R.sub.j (i), I.sub.j (i); i=1, . . . , n}.sub.t=ti
are read out from the memories 122 and 124. And in the complex auto-correlation arithmetic unit 128, the complex auto-correlation value Cora (j, t.sub.i) as to the scan line j and depth t.sub.i is calculated on the basis of the following equation (1) ##EQU1##
Incidentally, it is noted that the same reference symbol is used for the signals before and after the input and output of the MTI filter 126.
The complex auto-correlation value Cor(j, t.sub.i) calculated on the basis of the above-noted equation (1) is applied to the atan arithmetic unit 130. In the atan arithmetic unit 130, a phase difference .DELTA..theta.(j, t.sub.i) involved in the scan line j and depth t.sub.i is calculated on the basis of the received complex auto-correlation value Cor(j, t.sub.i) in accordance with the following equation (2): EQU .DELTA..theta.(j, t.sub.i)=a tan [Im{Cor(j, t.sub.i)}/Real {Cor(j, t.sub.i)}] (2)
From the thus obtained phase difference .DELTA..theta.(j, t.sub.i), it is possible to calculate a velocity V(j, t.sub.i) involved in the scan line j and depth t.sub.i in accordance with the following equation (3): EQU V(j, t.sub.i)=(.DELTA..theta.(j, t.sub.i).multidot.C)/(4.pi.f.sub.o T)(3)
where
C: an ultrasonic velocity PA1 f.sub.o : a center-frequency of ultrasound PA1 T: a repetitive period of transmission and reception of ultrasound PA1 C: an ultrasonic velocity (usually, 1540 m/sec.) PA1 .omega..sub.o : a reference angular frequency of a signal used in the quadrature detecting circuit 204 (cf. FIG. 41) PA1 c: an ultrasonic velocity (usually, 1540 m/sec.) PA1 j: j-th sampling point regarding the z direction (depth direction) PA1 .DELTA.z: an interval between two sampling points aligned in the z direction PA1 a transmitter-receiver for transmitting ultrasonic waves each in a scan line direction within a subject a plurality of number of times and receiving reflecting ultrasonic waves from an inside of the subject to generate received signals; PA1 a quadrature detector for obtaining a quadrature detecting output of each of the received signals; PA1 an auto-correlator for determining a complex auto-correlation value of the quadrature detecting output; PA1 a movement information detector for detecting blood flow information within the subject or information as to a movement of a tissue within the subject on the basis of the complex auto-correlation value determined by said auto-correlator; and PA1 an average arithmetic unit for determining an averaging value of the complex auto-correlation values on a plurality of points adjacent to a predetermined point within the subject, PA1 wherein said movement information detector determines the blood flow information within the subject or the information as to a movement of a tissue within the subject on the basis of said averaging value. PA1 means for displaying a tomographic image of a subject on the basis of a received signal carrying information as to ultrasonic reflection along a scan line extending within the subject, said received signal being generated in such a manner that ultrasonic pulse beams are transmitted within the subject and reflecting ultrasonic waves from an inside of the subject are received by a plurality of ultrasonic transducers to perform a beamformation in phasing; PA1 means for detecting a movement inside the subject on the basis of a plurality of received signals each carrying information as to ultrasonic reflection along a same scan line, said plurality of received signals being generated in such a manner that ultrasonic pulse beams are repeatedly transmitted in a same direction within the subject a plurality of number of times; PA1 (2.sub.-- 1) complex signal conversion means for converting the received signals before or after the beamformation in phasing into a first complex signal using a predetermined reference frequency .omega..sub.o of reference signal; PA1 (2.sub.-- 2) time shift complex signal producing means for producing from said first complex signal a second complex signal and a third complex signal, said second complex signal and said third complex signal being mutually shifted by a predetermined time difference .DELTA..tau.; PA1 (2.sub.-- 3) complex correlation calculating means for calculating complex correlation value C.sub.i,i (t,.DELTA..tau.) at a time point of time t, wherein when ultrasonic pulse beams are repeatedly transmitted in a predetermined direction within the subject, a reference time point of i-th transmission of the ultrasonic pulse beam is selected as a starting point, of the second and third complex signals associated with the i-th transmission of the ultrasonic pulse beam in said predetermined direction, and additional complex correlation value C.sub.i,i+1 (t) at a time point of time t, wherein reference time points of i-th and (i+1)th of transmissions of the ultrasonic pulse beam in said predetermined direction are each selected as a starting point, of the second complex signal-to-second complex signal which are associated with the i-th and (i+1)th of transmissions of the ultrasonic pulse beam in said predetermined direction, respectively; and PA1 (2.sub.-- 4) movement quantity calculating means for calculating on the basis of the complex correlation values C.sub.i,i (t,.DELTA..tau.) and C.sub.i,i+1 (t) calculated by said complex correlation calculating means a quantity representative of a movement as to an observation point associated with the time t, within the subject, which movement will occur between the time points of i-th and (i+1)th of transmissions of the ultrasonic pulse beam. PA1 it is acceptable that the system further comprises means for performing a beamformation process in phasing for a plurality of analog received signals derived from a plurality of ultrasonic transducers, and PA1 said complex signal conversion means includes a quadrature detector for performing a quadrature detection for the analog received signals subjected to the beamformation process in phasing using two analog sine wave signals, which are mutually different in phase by 90.degree., as said reference signal, thereby converting said received signals into the first complex signal in the form of analog; PA1 it is acceptable that the system further comprises PA1 means for performing a beamformation process in phasing for a plurality of analog received signals derived from a plurality of Ultrasonic transducers, and an A/D converter for converting the analog received signals outputted from said beamformation process means into digital received signals, and PA1 said complex signal conversion means includes a quadrature detector for performing a quadrature detection for the digital received signals outputted from said A/D converter using two digital sine wave signals, which are mutually different in phase by 90.degree., as said reference signal, thereby converting said received signals into the first complex signal in the form of digital; PA1 it is acceptable that the system further comprises PA1 an A/D converter for converting a plurality of analog received signals derived from a plurality of ultrasonic transducers into a plurality of digital received signals, and means for performing a beamformation process in phasing for the plurality of digital received signals outputted from said A/D converter, and PA1 said complex signal conversion means includes a quadrature detector for performing a quadrature detection for the digital received signals outputted from said beamformation process means using two digital sine wave signals, which are mutually different in phase by 90.degree., as said reference signal, thereby converting said received signals into the first complex signal in the form of digital; or PA1 it is acceptable that said complex signal conversion means includes a quadrature detector for performing a quadrature detection for a plurality of analog received signals outputted from a plurality of ultrasonic transducers or a plurality of digital received signals obtained through an A/D conversion of the plurality of analog received signals using two analog or digital sine wave signals, which are mutually different in phase by 90.degree., as said reference signal, thereby converting said received signals into a plurality of the first complex signals, and PA1 said system further comprises means for performing a beamformation process in phasing for the plurality of the first complex signals derived by said complex signal conversion means, thereby obtaining the first complex signal subjected to the beamformation process in phasing. PA1 it is acceptable that said time shift complex signal producing means includes an A/D converter for converting real part and imaginary part of the first complex signal in the form of analog into digital signals, respectively, using a sampling clock consisting of clock pulses each having a time interval .DELTA..tau. corresponding to the time difference .DELTA..tau., wherein 1/N (N: integer) of each of the respective operation time intervals for sequentially calculating a quantity representative of said movement of a plurality of observation points aligned in said predetermined direction within the subject for each observation point is expressed by the time difference .DELTA..tau., thereby producing the second complex signal in the form of digital and the third complex signal in the form of digital. PA1 (3.sub.-- 1) complex signal conversion means for converting the received signal into a complex signal consisting of two signals which quadrate with each other; PA1 (3.sub.-- 2) quadratic complex auto-correlation arithmetic means for calculating quadratic complex auto-correlation values of said complex signal-to-complex signal which are involved in transmissions of ultrasonic pulse beams at mutually different time points along a predetermined scan line, respectively, when ultrasonic pulse beams are repeatedly transmitted along the predetermined scan line extending a depth direction within the subject, and said complex signal-to-complex signal which are involved in the associated ones of a plurality of depth positions on said scan line, respectively; and PA1 (3.sub.-- 3) velocity gradient calculating means for calculating a velocity gradient within the subject on the basis of said quadratic complex auto-correlation values. PA1 (3.sub.-- 4) said quadratic complex auto-correlation arithmetic means calculates complex auto-correlation values of said complex signal-to-complex signal which are associated with mutually different depth positions on the predetermined scan line, respectively, on each of said complex signals associated with transmissions of ultrasonic pulse beams at mutually different time points, and thereafter calculates complex auto-correlation values of said complex auto-correlation value-to-complex auto-correlation value associated with transmissions of ultrasonic pulse beams at mutually different time points; PA1 (3.sub.-- 5) said quadratic complex auto-correlation arithmetic means calculates complex auto-correlation values of said complex signal-to-complex signal which are involved in a same depth position, in transmissions of ultrasonic pulse beams at mutually different time points along the predetermined scan line, respectively, on each of a plurality of depth positions, and thereafter calculates complex auto-correlation values of said complex auto-correlation value-to-complex auto-correlation value, which are associated with mutually different depth positions; or PA1 (3.sub.-- 6) said quadratic complex auto-correlation arithmetic means performs simultaneously both the operations on a complex auto-correlation of complex signal-to-complex signal associated with mutually different transmissions and a complex auto-correlation of complex signal-to-complex signal associated with a depth direction. PA1 (3.sub.-- 8) it is also acceptable that the quadratic complex auto-correlation arithmetic means in item (3.sub.-- 2) calculates a plurality of quadratic complex auto-correlation values to determine the velocity gradient as to the single depth position. PA1 (3.sub.-- 9) the quadratic complex auto-correlation arithmetic means in item (3.sub.-- 2) calculates complex auto-correlation values of said complex signal-to-complex signal which are involved in a same depth position, in transmissions of ultrasonic pulse beams at mutually different time points along the predetermined scan line, respectively, on each of a plurality of depth positions, and thereafter calculates complex auto-correlation values of said complex auto-correlation value involved in a predetermined depth position and said complex auto-correlation values which are involved in associated ones of a plurality of depth positions, respectively. PA1 (3.sub.-- 10) the quadratic complex auto-correlation arithmetic means in item (3.sub.-- 2) calculates complex auto-correlation values of said complex signal-to-complex signal which are involved in a same depth position, in transmissions of ultrasonic pulse beams at mutually different time points along the predetermined scan line, respectively, on each of a plurality of depth positions, and thereafter performs a complex auto-correlation operation on a first set comprising said complex auto-correlation values associated with a plurality of depth positions, and a second set comprising said complex auto-correlation values associated with a plurality of depth positions, which are permitted to overlap said complex auto-correlation values constituting the first set. PA1 (3.sub.-- 11) the quadratic complex auto-correlation arithmetic means in item (3.sub.-- 2) is adapted for determining a plurality of said complex auto-correlation values with respect to a plurality of depth positions to evaluate a velocity gradient as to a predetermined depth position, and PA1 the velocity gradient calculating means in item (3.sub.-- 3) is adapted for determining phase messages involved in each of said plurality of said complex auto-correlation values to evaluate the velocity gradient as to said predetermined depth position through regression of a predetermined odd function to the determined phase messages. PA1 (3.sub.-- 12) the quadratic complex auto-correlation arithmetic means in item (3.sub.-- 2) is adapted for determining a plurality of said complex auto-correlation values with respect to a plurality of depth positions to evaluate a velocity gradient as to a predetermined depth position, and PA1 the velocity gradient calculating means in item (3.sub.-- 3) is adapted for calculating a complex auto-correlation value of said complex auto-correlation value-to-said complex auto-correlation value to evaluate the velocity gradient as to said predetermined depth position on the basis of the thus determined complex auto-correlation value. PA1 .omega..sub.o =2.pi.f.sub.o : reference angular frequency of quadrature detection PA1 h.sub.c (t): real part of a complex signal obtained by quadrature detection PA1 h.sub.s (t): imaginary part a complex signal obtained by quadrature detection ##EQU5##
The above operations are repeated while incrementing depth t.sub.i and scan line number j, whereby the blood flow velocity of the image on the screen is determined.
FIG. 39(D) plots the blood flow velocity calculated in the above-mentioned fashion in connection with a certain scan line.
According to the related art shown in FIG. 37, the signals stored in the memories 122 and 124 are read out and passed via the MTI filter 126, thereby extracting the blood flow information. However, it should be noticed, in case of detection of a movement of the tissue, that the signal component representative of information as to the movement of the tissue is extremely larger in power than the signal component representative of the blood flow information, and thus there is no need to eliminate the blood flow information. Hence, performing the above-mentioned operations with respect to the signal bypassed the MTI filter 126 makes it possible to see the mode of movement of the tissue.
Next, there will be explained further related art.
Japanese Patent Publication No. 44494/1987 discloses an ultrasonic doppler diagnostic system in which a velocity of a blood flow of the heart is displayed in the form of an image. According to such a system, reflecting ultrasounds from the inside of the subject are received, and a Doppler effect which the reflecting ultrasounds undergoes is detected as a shift of an ultrasonic carrier frequency. This detection technique is called a pulse-pair method.
FIG. 41 is a block diagram of the related art of ultrasonic doppler diagnostic system.
Since the general technical matters of the ultrasonic diagnostic system are well known, there will be mainly explained the matters associated with the pulse-pair method.
Pulse signals are transmitted from a transmission circuit 202 each in a predetermined timing toward ultrasonic transducers 201 arranged with a plurality of pieces, so that ultrasonic pulse beams are transmitted from the ultrasonic transducers 201 each in a predetermined direction inside the subject (not illustrated). The ultrasonic pulse beams irradiated within the subject are reflected within the subject, and the reflected ultrasounds are received by the ultrasonic transducers 201 and are applied to a receiving circuit 203. In the receiving circuit 203, the ultrasounds are subjected to an addition in phase to generate received signals each carrying information with respect to a predetermined direction within the subject. The received signal is subjected to the quadrature detection in a quadrature detecting circuit 204, thereby obtaining two signals h.sub.ci (t) and h.sub.si (t) which quadrate with each other, where the suffix "i" implies that the signal is involved in the i-th transmission when the ultrasonic pulse beam are repeatedly transmitted in a predetermined direction within the subject, and the alphabetical mark "t" is representative of time as the reference time on each transmission. These signals h.sub.ci (t) and h.sub.si (t) are considered as a complex signal [h.sub.ci (t)+j h.sub.si (t)] taking in their combination.
The real part h.sub.ci (t) and imaginary part h.sub.si (t) of the complex signal [h.sub.ci (t)+j h.sub.si (t)] are applied to A/D converters 205 and 206, respectively, in which they are subjected to a sampling process at regular intervals, converted into a digital complex signal, and then temporarily stored in a memory 207. The digital complex signal is read out from the memory 207 and then applied to an MTI filter 208. The MTI filter 208 is constituted of a low-frequency cut-off filter which serves to eliminate information as to a clutter component which the received complex signal carries and to extract blood flow information. The complex signal representative of the blood flow information, which is outputted from the MTI filter 208, is temporarily stored in a memory 209, and then applied to a complex correlation arithmetic unit 210.
In case of calculating a movement of the tissue, it is sufficient to bypass the MTI filter 208, or alter characteristics of the MTI filter 208. A signal intensity derived from the tissue is extremely larger in comparison with a signal intensity of a blood flow component. Thus, even if the blood flow information is not eliminated, the complex signal, which is deemed to be substantially representative of the reflecting signal from the tissue, may be applied to the complex correlation arithmetic unit 210.
The complex correlation arithmetic unit 210 performs an arithmetic operation for the complex correlation between a complex signal corresponding to the i-th of beam transmission and a complex signal corresponding to the (i+1)-th of beam transmission, so that a complex correlation value C.sub.i,i+1 (t) can be calculated. This complex correlation value C.sub.i,i+1 (t) is applied to a velocity arithmetic unit 211 and be converted into a blood flow velocity V inside the subject. The blood flow velocity V is applied to a digital scan converter (DSC) 212 and be converted into a display-oriented signal. In a display unit 213, the blood flow velocity distribution is superposed on a tomographic image within the subject and displayed in color for instance. Incidentally, a technique for obtaining the tomographic image is well known, and the present invention is not substantially related to the technique for obtaining the tomographic image. Thus, the explanation as to the technique for obtaining the tomographic image is omitted.
Now, it is assumed that the ultrasonic diagnostic system as shown in FIG. 41 is used to transmit ultrasound beams in a specified direction within the subject every repetitive period T, thereby obtaining the complex signal Z.sub.i =X.sub.1 +j Y.sub.i (i=1, 2, 3, . . . ) with respect to a certain observation point depth in the specified direction within the subject.
In this case, an expected value of the complex correlation derived from the complex correlation arithmetic unit 210 shown in FIG. 41 is expressed by: EQU &lt;Z.sub.i+1 Z.sub.i* &gt;=&lt;X.sub.1+1 X.sub.i +Y.sub.i+1 Y.sub.i &gt;+j&lt;Y.sub.i+1 X.sub.i -Y.sub.i+1 Y.sub.i &gt; (4)
From Equation (4), an expected value &lt;.DELTA..theta.&gt; of a phase difference .DELTA..theta. resulting between repetitive periods T is given by: EQU &lt;.DELTA..theta.&gt;=arc tan {&lt;Y.sub.i+1 X.sub.i -Y.sub.i+1 Y.sub.i &gt;/&lt;X.sub.i+1 X.sub.i +Y.sub.i+1 Y.sub.i &gt;} (5)
If the transmitted ultrasonic pulse is of a very narrow band, a relation between the detected phase difference &lt;.DELTA..theta.&gt; and a doppler frequency f.sub.d resulting from a movement (here, the presence of a blood flow) of the inside of the subject is given by the following equation: EQU &lt;.DELTA..theta.&gt;=2.pi.f.sub.d .multidot.T (6)
Further, a velocity V of the movement concerned (a blood flow velocity) is expressed by: EQU V=(c/2.omega..sub.o).multidot.&lt;.DELTA..theta.&gt;/T (7)
where
Next, there will be explained still further related art.
FIG. 42 is a block diagram of the related art of ultrasonic diagnostic system.
Pulse signals are transmitted from a transmission circuit 302 each in a predetermined timing toward ultrasonic transducers 301 arranged with a plurality of pieces, so that ultrasonic pulse beams are transmitted from the ultrasonic transducers 301 each along a scan line extending in a predetermined depth direction inside the subject (not illustrated). The ultrasonic pulse beams irradiated within the subject are reflected within the subject, and the reflected ultrasounds are received by the ultrasonic transducers 301 and are applied to a receiving circuit 303. In the receiving circuit 303, the ultrasounds are subjected to a beamformation to generate received signals each carrying information with respect to the associated scan line within the subject. The received signals are applied to a quadrature detecting circuit 304 and a detecting circuit 314.
In the detecting circuit 314, the received signal is detected. The detected signal is converted into a display signal in a digital scan converter (DSC) 312. In a display unit 313, a tomographic image within the subject is displayed. Incidentally, a technique for obtaining the tomographic image is well known, and the present invention is not substantially related to the technique for obtaining the tomographic image. Thus, the explanation as to the technique for obtaining the tomographic image is omitted.
On the other hand, the received signal applied to the quadrature detecting circuit 304 is inputted to mixers 304a and 304b which constitute the quadrature detecting circuit 304. Also inputted to the mixers 304a and 304b are reference signals cos .omega..sub.o t and sin .omega..sub.o t from a control signal generating unit 305, respectively, which are multiplied by the received signal. Where .omega..sub.o denotes a reference angular frequency. The signals outputted from the mixers 304a and 304b are applied to low-pass filters 304c and 304d, respectively, in which signals involved in the low frequency band are extracted. The signals outputted from the low-pass filters 304c and 304d are converted into digital signals A/D converters 306a and 306b, thereby obtaining two signals R.sub.i (t) and I.sub.i (t) which quadrate with each other, where the suffix "i" implies that the signal is involved in the i-th transmission when the ultrasonic pulse beam are repeatedly transmitted in a predetermined direction within the subject, and the alphabetical mark "t" is representative of time as the reference time on each transmission. These signals R.sub.i (t) and I.sub.i (t) are considered as a complex signal [R.sub.i (t)+j I.sub.i (t)] when taking in their combination.
The real part R.sub.i (t) and imaginary part I.sub.i (t) of the complex signal [R.sub.i (t)+j I.sub.i (t)] are temporarily stored in a memory 307. The digital complex signal is read out from the memory 307 and then applied to a clutter eliminating means 308 such as an MTI filter. The clutter eliminating means 308 is constituted of a low-frequency cut-off filter or the like which serves to eliminate information as to a clutter component which the received complex signal carries and to extract blood flow information. In order to calculate a blood flow velocity, it is necessary to eliminate information as to a clutter component and extract only blood flow information. However, since a signal intensity derived from the tissue is extremely larger in comparison with a signal intensity of a blood flow component, it is usual for a calculation of the velocity of the tissue or the like not to eliminate the blood flow information.
The complex signal read out from the memory 307 is passed via a switch 309 to a complex auto-correlation arithmetic unit 310, after passing through the clutter eliminating means 308 or not passing through the clutter eliminating means 308 depending on the situation as to whether a blood flow velocity is to be detected or a tissue velocity is to be detected.
The complex correlation arithmetic unit 310 performs an arithmetic operation for the complex auto-correlation between a complex signal corresponding to the i-th of beam transmission and a complex signal corresponding to the (i+1)-th of beam transmission with respect to the same direction within the subject, so that a complex auto-correlation value C.sub.i,i+1 (t) can be calculated. Incidentally, the symbol mark &lt; . . . &gt; in the figure denotes an equalizing operation (expected value). This complex correlation value C.sub.i,i+1 (t) is applied to a velocity and velocity gradient arithmetic unit 311 and be converted into a blood flow velocity or a tissue velocity inside the subject or a velocity gradient.
The detected blood flow velocity or velocity gradient is applied to a digital scan converter (DSC) 312 and be converted into a display-oriented signal. In a display unit 313, the velocity distribution or the velocity gradient distribution is changed over in a display mode from a display of a tomographic image within the subject, or is superposed on the tomographic image and displayed in color for instance. Now, it is assumed that the ultrasonic diagnostic system as shown in FIG. 42 is used to transmit ultrasound beams in a specified direction within the subject every repetitive period T, thereby obtaining the complex signal Z.sub.i =X.sub.i +j Y.sub.i (i=1, 2, 3, . . . ) with respect to a certain depth position (observation point) on a specified scan direction extended inside the subject.
In this case, an expected value &lt;C.sub.i,i+1 &gt; of the complex auto-correlation derived from the complex auto-correlation arithmetic unit 310 shown in FIG. 42 is expressed by: ##EQU2##
where the mark * denotes a complex conjugate
From Equation (8), an expected value &lt;.DELTA..theta.&gt; of a phase difference .DELTA..theta. resulting between repetitive periods T is given by: EQU &lt;.DELTA..theta.&gt;=arc tan {&lt;Y.sub.i+1 X.sub.i -X.sub.i+1 Y.sub.i &gt;/&lt;X.sub.i+1 X.sub.i +Y.sub.i+1 Y.sub.i &gt;} (9)
A relation between the detected phase difference &lt;.DELTA..theta.&gt; and a doppler frequency f.sub.d is given by the following equation: EQU &lt;.DELTA..theta.&gt;=2.pi.f.sub.d .multidot.T (10)
A maximum doppler frequency f.sub.dmax is expressed by EQU f.sub.dmax =1/(2T) (11)
Further, a velocity V of the movement concerned (a blood flow velocity, or a velocity of the tissue) is expressed by: EQU V=(c/2.omega..sub.o).multidot.&lt;.DELTA..theta.&gt; (12)
where
According to a technique for determining a velocity gradient, which is disclosed in Japanese Patent Publication No. 43381/1993, the velocity gradient is determined by differentiating (including differencing) the velocity V, which is calculated on the basis of equation (12), with respect to a depth direction z within the subject, that is, EQU dV/dz=(1/.DELTA.z) (V.sub.j+1 -V.sub.j) (13)
where
When times of received signals of ultrasound beams reflected from the two sampling points j and j+1 are expressed by t.sub.j and t.sub.j+1, respectively, .DELTA.t=t.sub.j+1 -t.sub.j represents time required for the ultrasound beams to go and back between the two sampling points j and j+1. Consequently, the following relation is set up between the interval .DELTA.z and the time .DELTA.t: EQU .DELTA.z=c.multidot..DELTA.t/2 (14)
Now let us calculate data collecting time required for producing one picture plane portion of data.
Assuming that a repetitive period T is 200 .mu.sec, the number of scan lines is 64 pieces, and the number of times of transmission regarding the same direction is 9 times (once for use in a B-mode image, and eight times for use in a doppler detection), time required for constituting a frame is given by: EQU 200 .mu.sec.times.9.times.64 =115.2 msec
This assumes about 8 (1/115.2 msec=8.6) in the frame rate. This frame rate is not sufficient for a diagnosis. On the other hand, if the number of times of repetition regarding the same direction is reduced for the purpose of enhancing the frame rate, this involves a problem concerning precision.
A point for enhancing the frame rate resides in the matter that the number of times of transmission regarding the same direction is reduced. Reducing the number of times of transmission regarding the same direction allows variance of the complex auto-correlation value used for detection of a velocity to be large, and thus resulting a degradation of precision of the finally obtained velocity V. However, it is considered that the degradation of precision is avoided by using also the complex auto-correlation values derived from the adjacent scan line and the adjacent depth, as shown in FIG. 38.
FIG. 43 is a block diagram of the related art of ultrasonic diagnostic system taking account of the above-mentioned consideration. Only the differences from the system shown in FIG. 37 will be described.
The ultrasonic diagnostic system shown in FIG. 43 has an average processing unit 131 after an atan arithmetic unit 130. The average processing unit 131 is arranged to perform an average processing for the respective velocities V(j, t.sub.i) involved in each associated scan line j and the associated depth t.sub.i, which are calculated in the atan arithmetic unit 130, as shown in FIGS. 38(A)-38(C), with respect to the adjacent scan line direction (FIG. 38(A)), the depth direction (FIG. 38(B)) on a single scan line, or both the scan line direction and the depth direction (FIG. 38(C)). In this fashion, it is considered that the detection precision is enhanced without lowering the frame rates.
However, the average processing scheme as shown in FIG. 43 is still insufficient as to enhancement of the detection precision, and thus further enhancement of the detection precision is desired.
Accordingly, in view of the foregoing, it is the first object of the present invention to enhance the detection precision without lowering the frame rates, or to enhance the frame rates without lowering the precision.
Further, according to the related art of ultrasonic diagnostic system, as described in reference to FIG. 41, information as to a movement of the inside of the subject, such as a blood flow velocity is derived on the basis a principle of the pulse-pair method. However, it should be noted that actually, the received signals as reflection signals from the random structure include such a large error that the phase difference is changed at random or the presumed velocity is provided with an offset owing to an influence of attenuation.
Hence, in order to suppress such an error, there are attempts to obtain information as to a movement of the inside of the subject with greater accuracy. One of such attempts is proposed in Japanese Patent Laid Open Gazette No. 286751/1991.
FIG. 44 is a circuit block diagram useful for understanding a further developed technique from the above-referenced proposal.
In FIG. 44, the same parts are denoted by the same reference numbers as those of FIG. 41, and redundant description will be omitted.
The received signal outputted from the reception circuit 203 is applied to a quadrature detecting circuit 204.sub.-- 1 and therein converted into complex signals. The complex signals are applied to A/D converter circuits 205.sub.-- 1 and 206.sub.-- 1 therein converted into digital complex signals, respectively. These digital complex signals are applied to a phase difference calculating unit 220. Further, the received signal outputted from the reception circuit 203 is passed via an analog delay line 212, through which the signal is delayed by delay time .DELTA..tau., to a quadrature detecting circuit 204.sub.-- 2 and therein converted into complex signals. The complex signals are applied to A/D converter circuits 205.sub.-- 2 and 206.sub.-- 2 therein converted into digital complex signals, respectively. These digital complex signals are applied to the phase difference calculating unit 220. What is meant by providing a delay for the received signal by the delay line 212 is to simulate such a Situation that the tissue along the direction in which ultrasonic pulse beams are transmitted is uniformly shifted by the corresponding delay time .DELTA..tau..
The phase difference calculating unit 220 calculates a phase difference .DELTA..theta..sub.r between two complex signals derived from an originally single received signal, and passes the calculated phase difference .DELTA..theta..sub.r to a memory 221.
The phase difference calculating unit 220 calculates also a phase difference .DELTA..theta..sub.1,2 between a received signal involved in transmission of the first time of ultrasonic pulse beam and a received signal involved in transmission of the second time of ultrasonic pulse beam, and passes the calculated phase difference .DELTA..theta..sub.1,2 to the memory 221. This phase difference .DELTA..theta..sub.1,2 is equivalent to the phase difference, as described referring to FIG. 41, which remarkably varies depending on the random structure inside the subject.
Thereafter, the phase difference .DELTA..theta..sub.r is read out from the memory 221 and is passed to a correction factor calculating unit 222 in which a correction factor .DELTA..tau./.DELTA..theta..sub.r is calculated and then passed to a correction arithmetic unit 223. Also the phase difference .DELTA..theta..sub.1,2 is read out from the memory 221 and passed directly to the correction arithmetic unit 223.
In the correction arithmetic unit 223, the received phase difference .DELTA..theta..sub.1,2 is multiplied by the correction factor .DELTA..tau./.DELTA..theta..sub.r to calculate a high precision of time difference .DELTA..theta..sub.1,2 .multidot..DELTA..tau./.DELTA..theta..sub.r. This time difference .DELTA..theta..sub.1,2 .multidot..DELTA..tau./.DELTA..theta..sub.r is passed a tissue parameter calculating unit 224 in which tissue parameters each representative of a degree of hardness of the associated point inside the subject are calculated on the basis of the time differences .DELTA..theta..sub.1,2 .multidot..DELTA..tau./.DELTA..theta..sub.r which have been calculated with regard to the respective points inside the subject. The thus obtained tissue parameters are supplied to a display unit 125 and displayed thereon.
The time differences .DELTA..theta..sub.1,2 .multidot..DELTA..tau./.DELTA..theta..sub.r will be described more in details hereinafter.
As mentioned above, the phase difference .DELTA..theta..sub.r denotes a phase difference between two received signals which are obtained in such a manner that a single received signal is divided into two systems and a signal associated with one of which systems is delayed by delay time .DELTA..tau., so that the two received signals can be obtained. That is, the phase difference .DELTA..theta..sub.r includes errors similar to the phase difference .DELTA..theta..sub.1,2. Consequently, if the phase variation is smooth and thus can be adequately approximated by a linear expression, in other words, if the phase difference .DELTA..theta..sub.r is proportional to the delay time .DELTA..tau., multiplying the known delay time .DELTA..tau. by the proportional amount .DELTA..theta..sub.1,2 /.DELTA..theta..sub.r of those phase differences .DELTA..theta..sub.1,2 and .DELTA..theta..sub.r makes it possible to exactly determine the time difference .DELTA..theta..sub.1,2 .multidot..DELTA..tau./.DELTA..theta..sub.r corresponding to the movement amount of the subject.
Using a technique explained referring to FIG. 44 makes it possible to correct the errors within a limit that the linear approximation is valid. However, it is necessary for such a technique to provide an extremely exact delay time .DELTA..tau., and thus there is a need to provide a high precision of analog delay line 212. This is a cause of increasing the cost of the system. Further, according to such a system, there are needed the two-system corresponding of quadrature detecting circuits and the like, and thus a circuit scale will be enlarged. This will be also a cause of increasing the cost of the diagnostic system, and further will be contrary to contributing miniaturization of the diagnostic system.
The above-referenced Japanese Patent Laid Open Gazette No. 286751/1991 discloses a technique in which there is provided only a single system, but not two systems each including a quadrature detecting circuit and the like as shown in FIG. 44, to perform a switching between passing through the delay line 212 and not passing through the delay line 212. In this case, since there is frequent such a case that the subject moves during the repetitive period, an effect of the correction is little. Further, there is proposed also a scheme such that instead of providing a delay line, timing of a transmission of the ultrasonic pulse beam is varied by an amount corresponding to the delay time .DELTA..tau. every repetition. Also in this scheme, however, an effect of the correction will be lost.
According to the Japanese Patent Laid Open Gazette No. 286751/1991, to determine the phase difference .DELTA..theta..sub.r between the received signals one of which undergoes delay and the other no delay, first, the phases of the respective signals are detected, and then taking into account a so-called wrap around (jumping in phase from .pi. to -.pi.), the difference (phase difference .DELTA..theta..sub.r) between phase-to-phase is calculated. However, the usual ultrasonic received signal involves frequent wrap arounds, and thus it is troublesome to conduct the correction.
In view of the foregoing, it is the second object of the present invention to provide an ultrasonic diagnostic system capable of knowing a movement inside the subject with high precision using a circuit structure which is relatively simple in arrangement.
Further, according to the related art of ultrasonic diagnostic system, as described above, the velocity V is determined on the basis of equation (12) and the obtained velocity V is differentiated with regard to the depth direction (z direction) as shown in equation (13), thereby calculating the velocity gradient dV/dz for the time being. However, in a case where a movement inside the subject is fast and there occurs a doppler shift exceeding one which causes a maximum drive frequency f.sub.dmax shown in equation (11), this involves such a problem that an error appears on the calculated value.
FIGS. 45(A)-(D) are each a view for use in explanation of the problem as mentioned above.
It is assumed that when phase differences, which are to be essentially detected with respect to two points A and B each in a certain depth, are expressed by .DELTA..theta..sub.A and .DELTA..theta..sub.B, respectively, as shown in FIG. 45(A), the phase difference .DELTA..theta..sub.A is within .pi., and the phase difference .DELTA..theta..sub.B is out of .pi.. By the way, the phase difference is simply permitted to be identified only between -.pi. and .pi.. Consequently, as shown in FIG. 45(B), regarding .DELTA..theta..sub.A, the value near .pi. is detected. On the other hand, regarding .DELTA..theta..sub.B, the value near -.pi. is detected. This causes a problem of the wrap around such that the phase jumps by 2.pi.. The above-noted equation (12) represents that the phase difference .DELTA..theta. is proportional to the velocity V, and also the velocity V, which is determined in accordance with the equation (12), assumes values as shown in FIG. 45(B). Consequently, it happens that the velocity gradient dV/dz, which is calculated on the basis of the equation (13), includes a large error that is far apart from the actual velocity gradient (FIG. 45(D) ), as shown in FIG. 45(C).
In view of the foregoing, it is the third object of the present invention to provide an ultrasonic diagnostic system capable of calculating the velocity gradient with great accuracy, without involving a problem of the wrap around.